The ever increasing demands for high capacity communications systems has resulted in a wide spread deployment of optical fiber networks across the world. A fundamental component used in such systems receives pulses of light and converts these into electrical signals. The pulses of light in such systems comprise a bit stream of information. This fundamental component employed in the fiber optic networks is commonly known as an optical receiver module. Within the optical receiver, a photodetector is typically employed to receive the light pulses and an amplifying circuit is employed for amplifying photocurrent generated within the photodetector.
Transimpedance amplifiers (TIAs) are typically used within optical receiver modules to amplify and transform weak photocurrents received from the photodetector, in the form of a photodiode or a PIN diode. The TIA transforms the photocurrent into an output voltage that is further provided to other stages of the optical receiver module. Since TIAs are used to deal with both strong and weak photocurrents, noise in the resultant amplification and transformation to a voltage signal is typically a problem. Indeed, for those skilled in the art of the design of TIAs, it is well understood and appreciated that the noise introduced by the TIA, in many circumstances, limits the ability of the optical receiver module to faithfully reconstruct the intended stream of information. Furthermore, a relationship between the rate at which errors are produced by the receiver—often called the Bit Error Rate (BER), and the noise generated by the TIA can be shown. Thus, the optical receiver module needs to have low noise amplification performed on the weak photocurrents in order to facilitate optical transmission of information. This is especially true in circumstances where the distance that the optical signal must travel is long and results in weak optical pulses at the receiver. It is known to those skilled in the art that long transmission distances—the distance between a transmitter and a receiver—serves to attenuate the initial transmitted optical signal strength and places a greater burden upon the receiver module to avoid errors. Furthermore, it is also known that cost of an optical communication system is reduced if a signal is transmitted along a longer length of optical fiber or, in the alternative, if less optical power is transmitted. Thus, providing low noise amplification for the TIA is important in order to reduce the bit error rate (BER) of the received and amplified signal.
However, if the power supply noise is present, this can appear at the output port of the TIA in the form of an interference signal along with output noise of the TIA. The resulting effect is to cause an optical penalty of the TIA with the effect of reducing optical sensitivity. Therefore, to preserve the low noise properties of the TIA, a high power supply rejection (PSR) is typically required at frequencies below 100 KHz. For frequencies above 100 KHz, off-chip power supply filters are typically used.
Most amplifiers circuits offer a differential input signal and output signal mode of operation and as such the differential amplifier circuits are designed to have high gain and high common mode rejection. Unfortunately, the main issue with a transimpedance amplifier (TIA) is that it has a single input port and a dual output port, and the output ports are often required to be of a differential type in order to interface with a differential input post amplifier circuit. As a result, the conversion from single input port operation to a dual output port configuration for differential operation is often the cause of poor power supply rejection.
A need therefore exists to provide a power supply noise rejection circuit for use with a TIA that allows for a reduction in amplifier output signal noise. It is therefore an object of the invention to provide a power supply noise rejection circuit for use with a TIA that provides power supply noise rejection using a reduced amount of circuitry, thus reducing power dissipation and semiconductor circuit area.